perf stat: Move metric list from config to evlist

perf stat: Move metric list from config to evlist

The rblist of metric_event that then have a list of associated
metric_expr is moved out of the stat_config and into the evlist. This
is done as part of refactoring things for python, having the state
split in two places complicates that implementation. The evlist is
doing the harder work of enabling and disabling events, the metrics
are needed to compute a value and it doesn't seem unreasonable to hang
them from the evlist.

Signed-off-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20250710235126.1086011-7-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>